Radioactivities in returned lunar materials

The Ar37, Ar39, and H3 were measured at four depths (from 0 to 19.5 cm) of the deep core from Apollo 16 and in four other Apollo 16 samples. The Ar37 increased steadily from 40 dpm/kg at the top of the core to 68 dpm/kg at 19-cm depth. The comparison of the Ar37 in the core with that in rock 15555 shows that the solar flare at the time of the Apollo 16 mission was approximately an order of magnitude less intense than solar flares of 24 January 1971 and 2 November 1969, which occurred before the Apollo 14 and 12 missions. The Ar39 activities in the top 19 cm of the deep core varied little with depth. Because the Apollo 16 samples have a much higher Ca content and much lower Fe and Ti contents than do the documented rocks from previous missions, the Ar39 in the Fe, Ca, and K can be determined from Ar39 measurements on lunar material if a Ti cross section is assumed.